The subject matter of the present invention relates generally to an optical dimension measuring apparatus for sensing two opposite edges of an object to measure the dimensions of such object without contacting it. In particular, the present invention relates to an optical extensometer that scans the object with sequentially pulsed light beams of different light characteristics which are time modulated in an alternating manner. The pulsed light beams produce a measurement signal with a binary level transition when the beams cross the opposite edges of the object to precisely locate such edges.
The optical extensometer of the present invention is especially useful in making accurate high speed measurements of objects under test conditions or at remote locations where it is not possible to contact the object. For example, it can be used as a non-contacting strain gauge for measuring dimensional changes of a sample in an environmental chamber while the sample is heat cycled or mechanically stressed at high rates. Another use is to gauge the dimensions of objects in radioactive radiation cells, wind tunnels or other environmental chambers during varying thermal or atmospheric conditions. The optical extensometer can be used to measure the dimensions of small delicate objects, such as integrated circuits, or large objects such as logs and lumber.
It has previously been proposed to provide an optical dimension measuring apparatus for determining the size of a sample by measuring the time it takes to scan a laser light beam between the opposite edges of such object. If the scanning velocity of the light beam is known and the scanning time measured, the dimension of the object can be determined by multiplying these two factors together. However, such prior measurement system is not sufficiently accurate due to changes in the scanning velocity, instability in the laser light output instability in the gain of the photodetection system or in the threshhold reference voltage of the comparator circuit connected to the output of the photodetector. One such system using a single scanned light beam is described in the article, "Laser Extensometer," by W. J. Coleman, Instrumentation Technology Journal of Instrument Society of America, January 1967. Another similar system using a dithered, pulsed light beam is shown in the article "Rapid, Precise Computer-Controlled Measurement of X-Y Coordinates," by M. Kallmeyer et al, IBM J. Res. Develop., November 1973, pp. 490-499. This system does not use a phase comparator but differentiates and then zero detects the measurement signal.
The optical extensometer of the present invention overcomes these disadvantages and has greater measurement accuracy, resolution, repeatability and range. Thus, one embodiment of the invention with a beam diameter of 0.002 inch has a measurement accuracy of .+-.20 .times. 10.sup.-6 inch.
In one embodiment of the present invention the extensometer is capable of measuring dimensional changes along two mutually perpendicular axes, and with slight modifications, can also be made to sense along a third axis to indicate three dimensional changes in objects.
Other optical dimensional measuring apparatus employing a pair of scanned light beams are shown in U.S. Pat. No. 3,853,406 of Zanoni granted Dec. 10, 1974 and U.S. Pat. No. 3,740,152 of Iisuka, granted June 19, 1973. However, in neither of these patents is a pair of pulsed or time modulated light beams employed. Also, none of the above discussed referennces use three beam segments of two different light characteristics which are symmetrically deposed with two segments of one characteristic on opposite sides of a middle segment of the other characteristic, and are scanned sequentially across the opposite edges of the object to produce a measurement signal which changes polarity at such edges in the manner of the present invention. In the Zanoni patent, the light beams are continuous, not pulsed, and are spaced apart a known distance so that they sequentially scan the object. Also, the opposite edges of the object are not determined by phase comparison of the measurement signal with a reference signal, but instead are determined by first and second differentiation of the measurement signal. The Iisuka patent does show the use of a phase detector in an optical dimensional measuring apparatus, but does not employ pulsed light beams of different light characteristics and does not scan the beams across the edges of an object. Instead, it employs a double image optical sysem and transmits a real double image of the object through a vibrated scanning mask which spacially modulates the light image, rather than time modulated or pulsing the light beam. U.S. Pat. No. 3,907,400 of W. Eichenberger et al, granted Sept. 23, 1975, is of interest because it shows a thread monitoring system using a pair of sequentially pulsed light beams. However, such light beams are not scanned across the opposite edges of an object to measure a dimension of such object.